Waveguide modulator structures

ABSTRACT

An optoelectronic device and method of making the same. In some embodiments, the optoelectronic device includes a substrate, a Mach-Zehnder waveguide modulator, and an epitaxial crystalline cladding layer. The Mach-Zehnder waveguide modulator includes a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide, each of the left and right optical waveguides including a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion. The epitaxial crystalline cladding layer is on top of the substrate and beneath the junction region of the left optical waveguide and/or the junction region of the right optical waveguide, and has a refractive index which is less than a refractive index of the respective junction region(s), such that optical power is confined to the respective junction region(s).

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation-in-part of PCT Application PCT/EP2017/080216, filed Nov. 23, 2017, and a continuation-in-part of U.S. patent application Ser. No. 15/555,431, filed Sep. 1, 2017; PCT Application PCT/EP2017/080216 claims priority to U.S. Provisional Patent Application 62/426,117, filed Nov. 23, 2016, to U.S. Provisional Patent Application 62/427,132, filed Nov. 28, 2016, to United Kingdom Patent Application GB1711525.4, filed Jul. 18, 2017, to U.S. patent application Ser. No. 15/700,053, filed Sep. 8, 2017, and to U.S. patent application Ser. No. 15/700,055, filed Sep. 8, 2017; British Patent Application GB1711525.4 claims priority to U.S. Provisional Patent Application 62/528,900, filed Jul. 5, 2017; U.S. patent application Ser. No. 15/700,053 claims the benefit of U.S. Provisional Patent Application 62/528,900, filed Jul. 5, 2017; U.S. patent application Ser. No. 15/700,055 claims the benefit of U.S. Provisional Patent Application 62/528,900, filed Jul. 5, 2017; U.S. patent application Ser. No. 15/555,431 is a national stage application of PCT Application PCT/GB2016/050570, filed Mar. 4, 2016; PCT Application PCT/GB2016/050570 claims priority to U.S. Provisional Patent Application 62/128,949, filed Mar. 5, 2015, and to U.S. Provisional Patent Application 62/152,696, filed Apr. 24, 2015. The entire contents of all documents identified in this paragraph are hereby incorporated herein by reference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to optoelectronic devices, and particularly to optoelectronic devices with no buried oxide layer or other insulating layer between an optically active region and a substrate.

BACKGROUND

Conventional optoelectronic devices (for example, electro-absorption modulators or EAMs) comprise a waveguide on a base that is generally a semiconductor substrate such as a silicon substrate. Waveguides built upon this base comprise three layers: a core layer, a bottom cladding layer, and an upper cladding layer; which are configured to guide a light signal through the core layer by total internal reflection. The core layer is a light-transmitting medium, which is conventionally a thin (relative to an active material discussed below) silicon layer located on top of an insulating layer such as a buried oxide or BOX layer. The BOX layer, as part of the waveguide (bottom cladding) is located on top of the silicon substrate and functions to confine the light into the light-transmitting medium. An optoelectronic device may comprise a waveguide with an optically active region (also referred to as an active waveguide), for example an electro-absorption medium, deposited in a cavity in the silicon layer (i.e. atop the BOX layer). Typically, a thin silicon layer is left on the bottom of the cavity between the BOX layer and the optically active region as a crystal seed for the active material to be grown epitaxially. Both the silicon seed layer and the BOX layer may function as the bottom cladding for the active waveguide. Usually, the epitaxial growth for the active material needs a further active material seed layer located upon the silicon seed layer in order to obtain a high quality crystal structure of the desired active region. For example, a seed layer of germanium may be grown when an active layer of silicon-germanium is to be grown. The uniform and continuous silicon layer must be kept relatively thin so as to maintain the coupling efficiency between a passive waveguide (e.g. a non-optically active waveguide) and the active waveguide at a useful level. In previous optoelectronic devices, the thickness of the silicon layer is around 0.2 μm.

To fabricate a known EAM as described above from a silicon wafer, the silicon layer above the BOX must be etched to a thickness of around 0.2 μm from an initial thickness of around 3 μm. It is difficult to do this consistently, and so problems with yield may arise.

Known optoelectronic devices which operate at 1310 nm wavelengths suffer from a number of issues. For example, in Mach-Zehnder interferometer based devices operating at this wavelength have a very large footprint on a photonic circuit, which can result in a very large parasitic capacitance. Moreover, the driver circuit for such a device is very complicated and will often require a distributed electrode and transmission line design. Quantum-confined Stark effect devices operating at this wavelength show a high polarization dependency (which means that the performance difference such as insertion loss and extinction ratio for TE and TM modes can be out of the acceptable range) as well as a high sensitivity to manufacturing process tolerance. Operational bandwidth is also limited in a trade-off with extinction ratio i.e. for a given extinction ratio the maximum bandwidth is correspondingly limited.

Until now, it has been accepted that a silicon seed layer and a BOX layer are necessary beneath the optically active region as bottom cladding in order to make the optoelectronic device function. However, the inventors have realised that the silicon seed layer and the BOX layer are not necessary and can be replaced by other material that has a crystal structure with a lower refractive index than that of the optically active region.

SUMMARY

The invention generally relates to an optoelectronic device with a regrown or epitaxial crystalline cladding layer below an optically active region. For example, a silicon-on-insulator (SOI) wafer, where a portion of the buried oxide has been removed, and a cladding layer regrown in its place.

Accordingly, in a first aspect, the invention provides an optoelectronic device, comprising an optically active region and a substrate; wherein a bottom cladding layer positioned between the optically active region and the substrate, and directly adjacent to the substrate, is provided by an epitaxial crystalline layer. In this way, there is no buried oxide layer between the optically active region and the substrate. The substrate may be a silicon substrate.

The bottom cladding layer may be an epitaxial crystalline layer, for example a silicon or SiGe epitaxial cladding layer; and an upper surface of the epitaxial cladding layer may abut a bottom surface of the optically active region.

In a second aspect, the invention provides an optoelectronic device, comprising: a substrate; an epitaxial crystalline cladding layer, on top of the substrate; and an optically active region, above the epitaxial crystalline cladding layer; wherein the epitaxial crystalline cladding layer has a refractive index which is less than a refractive index of the optically active region, such that optical power of the optoelectronic device is confined to the optically active region.

The epitaxial crystalline cladding layer, may also be referred to as a regrown cladding layer or an epitaxial cladding layer or an epitaxial semiconductor layer. By regrown, it may be meant that during manufacturing, a pre-existing cladding layer may have been removed and a replacement regrown in its place. For example, when the pre-existing cladding layer is a buried oxide layer, it may be removed and a cladding layer may be regrown which is formed of a material which is not buried oxide. By epitaxial, it may be meant that a crystalline structure of the cladding layer is a function of a crystalline structure of the substrate upon which the cladding layer is located. The cladding layer may also be referred to as a crystalline cladding layer. By epitaxial crystalline, it may be meant that the cladding layer has been epitaxially grown using a semiconductor material. By the term optical power, it may be meant that an optical signal or a majority of the optical signal of the device is confined to the optically active region or that the main optical signal passing through the device passes through the optically active region.

Advantageously, the coupling loss between a passive waveguide (connected or connectable to the optically active region) and the optically active region according to the invention is smaller than that between the same regions in a prior art device that has a buried oxide layer. Generally, the cladding layer functions by having a contrasting (lower) refractive index than the active region and hence, the light is confined in the active region. Moreover, it is possible to tune the height of the cladding layer to optimize mode match which can result in lower device losses and higher yield in device fabrication. Furthermore, the absence of a buried oxide layer between the cladding layer and the optically active region can remove or diminish RF parasitic capacitance (e.g. by removal of C_(ox) capacitance) which may result in a higher device speed.

By regrown, it may be meant that the cladding layer is provided as a layer grown from the substrate or an intermediate layer e.g. it may be an epitaxial or crystalline cladding layer. The cladding layer may be directly on top of the substrate. Alternatively, the cladding layer may be separated from the substrate by a seed layer (for example, a germanium seed layer), or the cladding layer may function as a seed layer. The seed layer can help ensure that the crystal structure of the cladding layer and/or the optically active region is of good quality and have low defect density, as well as encouraging growth along the correct crystal axis. In some embodiments, the seed layer is kept thin, for example substantially equal to the thickness of the BOX in the passive waveguides, which is around 400 nm, as this will help minimise loss. Generally, the top surface of the seed layer should be substantially equal to the top surface of the BOX layer in the adjacent passive waveguides. The substrate may be a silicon substrate.

The optically active region may be one of: an electro-absorption modulator; a photodiode; or an avalanche photodiode. The electro-absorption modulator may operate by the Franz-Keldysh or Quantum-confined Stark effects. Where the optically active region is formed of any one of: SiGe, SiGeSn, GeSn, InGaNAs, or InGaNAsSb the optically active region may be an electro-absorption modulation operating by the Franz-Keldysh effect.

The device may further comprise an insulating layer, disposed on top of the substrate and horizontally adjacent to the cladding layer, and the cladding layer may be formed of a material which is different from the insulating layer. The insulating layer may be a buried oxide layer. By horizontally adjacent, it may be meant that the insulating layer is adjacent on a different side of the cladding layer than the substrate. Horizontal may refer to a direction perpendicular to a height of the device i.e. parallel to an upper surface thereof.

The cladding layer may be formed of silicon. The silicon may be epitaxially grown silicon. The cladding layer may be formed of a material which is not a buried oxide. The cladding layer may be formed from silicon germanium (SiGe), germanium (Ge), indium phosphide (InP), or may be a combination of a SiGe and Ge layer. Where there is a seed layer between the substrate and the cladding layer, the cladding layer may further act to isolate any optical losses which might be caused by the seed layer. The optically active region may be formed of SiGeSn, GeSn, InGaNAs, InGaNAsSb, or SiGe having a first composition and the cladding layer may be formed of the same material having a second composition different from the first composition. In this example, there may be a germanium seed layer or a SiGe layer, functional as a seed layer, disposed between the cladding layer and the silicon layer. Where a seed layer is provided between the cladding layer and the silicon substrate layer, the seed layer may sit directly on top of the silicon substrate layer and the cladding layer may sit directly on top of the seed layer. The optically active region may be formed of SiGeSn, and the cladding layer may be formed of SiGe.

The device may further comprise a germanium seed layer between the cladding layer and the optically active region. Alternatively, the seed layer may be SiGe or SiGeSn.

The device may further comprise an insulating layer (for example, a buried oxide layer), disposed on a first and/or second horizontal side of the cladding layer, wherein the cladding layer has a height from the substrate which is substantially equal to that of the insulating (e.g. buried oxide) layer. The cladding layer may have a height from the substrate (i.e. measured orthogonally to the plane of the substrate) which is 0.4 μm for a 3 μm silicon-on-insulator (SOI) platform, or which may be more for different SOI platforms. The cladding layer is formed of a material different from the buried oxide layer. The insulating (e.g. buried oxide) layer may extend under a portion of a slab of the optically active region i.e. the regrown or epitaxial crystalline cladding layer may not be as wide in a horizontal direction as the optically active region, the optically active region also comprising a rib under which the cladding layer is disposed.

The optically active region may be disposed within a cavity of a silicon-on-insulator layer which is disposed above the substrate. The optically active region may be formed of any of: SiGe, SiGeSn, an SiGe multiple quantum well (MQW) epitaxially grown stack, or an InP-based MQW epitaxially grown stack. The optically active region may be formed of Si_(x)Ge_(1-x-y)Sn_(y), where 5%≤x≤20% and 1%≤y≤10%. Such a device may be operable at an optical wavelength of 1310 nm, and may have a length of the optically active region of between 30 μm and 60 μm. The device may be driveable at a voltage of between 1.8 V-2 V which may be measured peak to peak and applied as a reversed bias. Advantageously, such a device is less polarization dependent than previous 1310 nm devices and the performance of the device is generally less sensitive to process variation.

The optically active region may be capped with a capping layer.

The device may further comprise an input waveguide, coupled to a first side of the optically active region; and an output waveguide, coupled to a second side of the optically active region; wherein the interface between the input waveguide and the optically active region and the interface between the output waveguide and the optically active region are at an angle greater than 0° relative to a guiding direction of the input waveguide and/or output waveguide.

The optically active region may include a SiGe optical waveguide, and the waveguide may comprise a junction region and a plurality of electrodes for providing a bias across the junction to enable use of the Franz-Keldysh effect, Quantum-confined Stark effect, or control of the phase of light traveling through the junction region via dispersion. The optically active region may include a SiGeSn optical waveguide, and the waveguide may comprise a junction region and a plurality of electrodes for providing a bias across the junction to enable use of the Franz-Keldysh effect or control of the phase of light traveling through the junction region via dispersion.

In an embodiment, the optically active region may include a waveguide ridge, and may have: an upper surface and a lower surface; a lower doped region, located at and/or adjacent to at least a portion of the lower surface of the optically active region, and extends laterally outwards from the waveguide ridge in a first direction; an upper doped region, located at and/or adjacent to at least a portion of the upper surface of the waveguide ridge of the optically active region, and extends laterally outwards from the waveguide ridge in a second direction; and an intrinsic region located between the lower doped region and the upper doped region.

In this embodiment, a first electrode may contact the lower doped region at a first contact surface and a second electrode may contact the upper doped region at a second contact surface; the first contact surface may be laterally offset from the waveguide ridge in a first direction; and the second contact surface may be laterally offset from the waveguide ridge in a second direction. The first and second contact surfaces may be aligned with one another along a lateral plane.

In this embodiment, the upper doped region may comprise a first doped zone and a second doped zone. The dopant concentration in the second doped zone of the upper doped region may be higher than the dopant concentration in the first doped zone of the upper doped region; and the second doped zone of the upper doped region may comprise the second contact surface. The first doped zone of the upper doped region may be at and/or adjacent to the upper surface of the waveguide ridge of the optically active region (OAR), and the second doped zone is located at a position which is laterally displaced from the waveguide ridge in the second direction.

In this embodiment, the lower doped region may comprise a first doped zone and a second doped zone. The dopant concentration in the second doped zone of the lower doped region may be higher than the dopant concentration in the first doped zone of the lower doped region; and the second doped zone of the lower doped region may comprise the first contact surface. The first doped zone of the lower doped region may be located directly underneath the OAR; and the second doped zone of the lower doped region may be located within the OAR, laterally displaced from the waveguide ridge, the second doped zone of the lower doped region may have an upper surface which comprises the first contact surface, and a lower surface which is in direct contact with the first doped zone of the lower doped region. The second doped zone of the lower doped region may be located within a portion of the OAR having a reduced height. The portion of the OAR having a reduced height may be a portion of the OAR which has been etched before the dopant species of the lower doped region is added.

In this embodiment. The first doped zone of the lower doped region may be located directly underneath the OAR. The OAR may include a slab which extends in the first direction, the slab may exhibit a via through its thickness at a location laterally displaced from the waveguide ridge in the first direction; and the second doped zone of the lower doped region may be located within the first doped zone, directly underneath the via.

In this embodiment, the lower doped region may be partially adjacent to the lower surface of the OAR and may be partially migrated into the OAR at the lower surface.

In this embodiment, the upper doped region may be fully located within the OAR.

In this embodiment, the OAR may be formed from an electro-absorption material in which the Franz-Keldysh effect occurs in response to the application of an applied electric field.

In this embodiment, the OAR may be formed from a light absorbing material which is suitable for generating a current upon detection of light and electrons to be swept out when a voltage bias is applied across the upper and lower doped regions.

In this embodiment, the optically active region may include a waveguide ridge, a first slab on a first side of the waveguide ridge and a second slab on a second side of the waveguide ridge, the OAR may have an upper and surface and a lower surface. The lower doped region may be located adjacent to a portion of a lower surface of the OAR; the lower doped portion may also extend laterally along and adjacent to the first slab of the OAR, away from the ridge in a first direction. The upper doped region may be located within at least a portion of an upper surface of the ridge of the OAR, and may extend laterally outwards along the second slab of the OAR in a second direction. The lower doped region, which may be located adjacent to a portion of a lower surface of the OAR, may migrate into the OAR at the same portion of the lower surface of the OAR.

In an embodiment, the optically active region may include a rib waveguide modulation region, the rib waveguide modulation region having: a ridge extending from the cladding layer; a first slab region at a first side of the ridge and a second slab region at a second side of the ridge; and wherein: a first doped region extends along: the first slab region and along a first side wall of the ridge, the first sidewall contacting the first slab region; and a second doped region extends along: the second slab region and along a second sidewall of the ridge, the second sidewall contacting the second slab region.

In a different embodiment, the optically active region may include a rib waveguide modulation region, the rib waveguide modulation region having: a ridge extending from the cladding layer, and all of the ridge, or at least a portion of the ridge being formed from a material which is different from the material of the cladding layer; wherein the rib waveguide modulation region includes a first slab region at a first side of the ridge and a second slab region at a second side of the ridge. A first doped region may extend along: the first slab region and along a first side wall of the ridge, the first sidewall contacting the first slab region. A second doped region may extend along: the second slab region and along a second sidewall of the ridge, the second side wall contacting the second slab region.

In an embodiment, the optically active region may include a rib waveguide modulation region, the rib waveguide modulation region having: a ridge extending from the cladding layer, at least a portion of the ridge being formed from a chosen semiconductor material which is different from the material of the cladding layer; a first slab region at a first side of the ridge and a second slab region at a second side of the ridge; and either the first slab region or the second slab region is the material of the cladding layer; and wherein: a first doped region extends along: the first slab region and along a first side wall of the ridge, the first sidewall contacting the first slab region; and a second doped region extends along: the second slab region and along a second sidewall of the ridge, the second sidewall contacting the second slab region.

In a different embodiment, the optically active region may further include a rib waveguide modulation region, the rib waveguide modulation region, the ridge waveguide modulation region having: a silicon base, disposed on top of the cladding layer; a ridge extending from the silicon base, at least a portion of the ridge being formed from a chosen semiconductor material which is different from the material of the silicon base. The silicon base may include a first slab region at a first side of the ridge, and a second slab region at a second side of the ridge. A first doped region may extend along: the first slab region and along a first sidewall of the ridge, the first sidewall contacting the first slab region. A second doped region may extend along: the second slab region and along a second sidewall of the ridge, the second sidewall contacting the second slab region.

In this, or the previous embodiment, the chosen semiconductor material may be silicon germanium (SiGe), silicon germanium tin (SiGeSn), GeSn, InGaNAs, InGaNAsSb, or a metal alloy of silicon, germanium or SiGe. The chosen semiconductor material may be a GeSn alloy grown on Si or on an III-V semiconductor.

In this, or the previous embodiment, the device may further comprise a first electrical contact located on the first slab region of the silicon base or cladding layer and a second electrical contact located on the second slab region of the silicon base or cladding layer. The first doped region may be n doped and the second doped region may be p doped.

In this embodiment, the ridge may comprise: a lower ridge portion in contact with and extending away from the base; the base and lower ridge portion being formed from silicon; and an upper ridge portion in contact with and extending away from the lower ridge portion, the upper ridge portion being formed from the chosen semiconductor material. The first doped region which extends along the first sidewall may include a lower sidewall portion located at the first ridge portion and an upper sidewall region located at the second ridge portion. The second doped region which extends along the second sidewall may include a lower sidewall portion located at the first ridge portion and an upper sidewall region located at the second ridge portion. The lower sidewall portions and slab regions may have a higher dopant concentration than the upper sidewall portions. The lower sidewall portions may have a higher dopant concentration than the upper sidewall dopant concentrations, and the slab regions of the doped regions may have a higher dopant concentration than the lower sidewall portion. A distance (d_(np2), d_(pp2)) by which the first and second lower sidewall portions extend into the ridge may be greater than the distance (d_(n), d_(p)) by which the first and second upper sidewall portions extend into the ridge.

In this, or the previous embodiment, the device may include an input rib waveguide coupled to the input of the rib waveguide modulation region to couple light into the rib waveguide modulation region, and an output rib waveguide coupled to the output of the rib waveguide modulation region to couple light out of the rib waveguide modulation region.

In this, or the previous embodiment, the height of the base or cladding layer (h₂) and the height of the lower sidewall portions (h₃) may be chosen such that the mode center of the rib waveguide is located at the same height above the base or cladding layer as the mode center of the input and/or output waveguide(s).

The optically active region may include a waveguide ridge; and a waveguide slab; and at least one of: a sidewall of the ridge; a portion of the slab; the entirety of the slab; a portion of the ridge adjacent to the slab; both sidewalls of the ridge; may be formed of crystalline or amorphous silicon and contains dopants. The remainder of the waveguide slab may be formed from SiGe, SiGeSn, GeSn, InGaNAs, InGaNAsSb or germanium.

In this embodiment, it may be that a portion of the slab and an adjacent sidewall are formed of crystalline or amorphous silicon. In such an example, the portion of the slab and the adjacent sidewall may contain dopants of a same species. In addition, it may be that a further portion of the slab and a further adjacent sidewall are formed of crystalline or amorphous silicon, and so both sidewalls and both portions of the slab either side of the ridge may be formed of crystalline or amorphous silicon. In this example, the further portion of the slab and the further adjacent sidewall contain dopants of a different species to those contained in the first portion of the slab and the first adjacent sidewall. Alternatively, the entire waveguide slab may be formed of crystalline or amorphous silicon, as well as a portion of the ridge which is adjacent to the slab, such that the crystalline or amorphous silicon forms an inverted ‘T’ shape. In such examples, a portion of the waveguide slab may contain dopants of a first species and a different portion of the waveguide slab may contain dopants of a second species. In addition to this, it may be that one or both sidewalls are formed of crystalline or amorphous silicon. In examples where only one sidewall is formed of crystalline or amorphous silicon (containing dopants), the other sidewall is formed from an active material (for example SiGe or SiGeSn) which is also doped. The sidewalls in such examples may contain dopants, and respectively contain dopants of a different species. Advantageously, such arrangements are easier to manufacture. Moreover, devices having at least one electrical contact disposed on a doped Si portion display improved series resistance which can improve the bandwidth and linearity of optical power.

In a third aspect, the invention provides an optoelectronic device, formed on a silicon-on-insulator wafer comprising a substrate, an insulating layer (e.g. a buried oxide layer), and a silicon-on-insulator layer, comprising: a cladding layer, formed of a different material to the material of the insulating layer, on top of the substrate; and an optically active region, above the cladding layer; wherein the cladding layer has a refractive index which is less than a refractive index of the optically active region such that an optical mode of the optoelectronic device is confined to the optically active region, and wherein the insulating layer does not extend below the optically active region. The optically active region may be formed of: SiGeSn, GeSn, InGaNAs, or InGaNAsSb. The optically active region may be formed of SiGe. The cladding layer may be formed of: silicon or SiGe.

The optoelectronic device of the third aspect may have any of the features discussed with relation to the optoelectronic device of the second aspect.

In a fourth aspect, the invention provides a method of manufacturing an optoelectronic device from a silicon-on-insulator wafer comprising a substrate, an insulating layer (e.g. a buried oxide layer), and a silicon-on-insulator layer, the method comprising the steps of: etching a cavity into the wafer, such that a depth of the cavity extends to at least an upper surface of the substrate; growing a cladding layer onto the upper surface of the substrate; growing an optically active material onto the cladding layer, wherein the cladding layer has a refractive index which is less than a refractive index of the optically active material; and etching the optically active material so as to form an optically active region above the cladding layer. The optically active material may comprise SiGe. The optically active material may comprise any one of: SiGeSn, GeSn, InGaNAs, or InGaNAsSb. The cladding layer may correspond to the cladding layer discussed above, and may have any of the features disclosed in relation thereto.

Advantageously, a better uniformity in slab height can be achieved due to there being less etch variability. Therefore the devices may be easier to manufacture and/or result in higher yields. Moreover, as the cladding layer is grown directly on the silicon substrate (or on a seed layer on the silicon substrate) the fabrication process can be faster than conventional methods. This can lead to a fabrication process which may result in higher density of devices on a chip, higher yield of devices produced, and a lower cost for integrating photonic circuits into a silicon-on-insulator platform.

The step of etching a cavity into the wafer may comprise: a first etching step, where the insulating (e.g. buried oxide) layer is used as an etch-stop; and a second etching step, where the substrate is used as an etch-stop.

The method may include a step, before growing the cladding layer, of growing a seed layer onto the upper surface of the substrate such that the cladding layer grows from the seed layer.

The method may include a further step after growing the optically active material of planarizing the grown optically active material.

The method may include a step, after growing the optically active material and before etching the optically active material, or disposing a hard mask on top of at least a part of the grown optically active region.

The method may include a step, after etching the optically active material, of: doping the optically active region with dopants of a first and second species, so as to provide an electro-absorption modulator.

In a fifth aspect, the invention provides an optoelectronic device comprising: a waveguide slab, disposed on top of an insulating layer (e.g. a buried oxide layer); and a waveguide ridge, disposed on top of the waveguide slab; wherein at least one of: a sidewall of the ridge; a portion of the slab; the entirety of the slab; a portion of the ridge adjacent to the slab; both sidewalls of the ridge; are formed of crystalline or amorphous silicon and contain dopants.

In this embodiment, it may be that a portion of the slab and an adjacent sidewall are formed of crystalline or amorphous silicon. In addition, it may be that a further portion of the slab and a further adjacent sidewall are formed of crystalline or amorphous silicon, and so both sidewalls and both portions of the slab either side of the ridge may be formed of crystalline or amorphous silicon. Alternatively, the entire waveguide slab may be formed of crystalline or amorphous silicon, as well as a portion of the ridge which is adjacent to the slab, such that the crystalline or amorphous silicon forms an inverted ‘T’ shape. In addition to this, it may be that one or both sidewalls are formed of crystalline or amorphous silicon.

A first sidewall and second sidewall of the waveguide ridge may be respectively doped with dopants of a first species and dopants of a second species. A first side of the waveguide slab, adjacent to the first sidewall, and a second side of the waveguide slab, adjacent to the second sidewall, may be respectively doped with dopants of a first species and dopants of a second species. The remainder of the waveguide ridge and waveguide slab may be formed of SiGe or germanium.

According to an embodiment of the present invention there is provided an optoelectronic device, including: a substrate; a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator including: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide, wherein each of the left and right optical waveguides includes a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion; and an epitaxial crystalline cladding layer, on top of the substrate and beneath the junction region of the left optical waveguide and/or the junction region of the right optical waveguide, wherein the epitaxial crystalline cladding layer has a refractive index which is less than a refractive index of the respective junction region(s), such that optical power is confined to the respective junction region(s).

In one embodiment, the junction region of each of the left and right optical waveguides includes a PIN region, the junction formed from: a first semiconductor region corresponding to either a P-doped region or an N-doped region; and a second semiconductor region corresponding to the other of the P-doped or N-doped region; and a central SiGe waveguide region; and a plurality of electrodes configured to apply a forward bias across one or both of the PIN junctions to enable control of the phase of light travelling through the respective PIN junction region(s) via dispersion.

In one embodiment, the first semiconductor region of the left optical waveguide is integral with the first semiconductor region of the right optical waveguide in a region between the left and right arms, forming a common doped region.

In one embodiment, the first semiconductor region of the left optical waveguide is electrically isolated from the first semiconductor region of the right optical waveguide by a central isolated region between the left and right arms.

In one embodiment, the junction of each of the left and right optical waveguides includes a PN junction, the PN junction formed from: a first semiconductor region corresponding to either a P-doped region or an N-doped region; a second semiconductor region corresponding to the other of the P-doped or N-doped region; and a plurality of electrodes configured to apply a reverse bias across one or both of the PN junctions to enable control of the phase of light travelling through the respective PN junction region(s) via dispersion.

In one embodiment, one of the first or second semiconductor regions includes a vertical doped portion which extends along the side of the waveguide.

In one embodiment, the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped region.

In one embodiment, the vertical doped portion extends along the entire side of the waveguide.

In one embodiment, the optoelectronic device includes an intervening lightly P-doped semiconductor region located between the P-doped semiconductor region and the central waveguide region; and the intervening lightly P-doped semiconductor region has a lower dopant concentration than the P-doped semiconductor region.

In one embodiment, the optoelectronic device includes an intervening lightly N-doped semiconductor region located between the N-doped semiconductor region and the central waveguide region and the intervening lightly N-doped semiconductor region has a lower dopant concentration than the N-doped semiconductor region.

In one embodiment, the central waveguide is composed of SiGe and the doped semiconductor regions are composed of Si or SiGe.

In one embodiment, the plurality of electrodes includes a common electrode located at the common doped region.

In one embodiment, for each waveguide arm: the first semiconductor region includes a first lateral portion extending laterally away from the waveguide wall on a first side of the waveguide; the second semiconductor region includes a second lateral portion extending laterally away from the waveguide wall on a second side of the waveguide; and the plurality of electrodes includes: a first electrode located directly on top of the first lateral portion; and a second electrode located directly on top of the first lateral portion.

In one embodiment, the optoelectronic device is configured to operate in a push-pull mode.

In one embodiment, the cladding layer is formed of silicon or SiGe.

In one embodiment, the optoelectronic device further includes an insulating layer, disposed on a first and/or second horizontal side of the cladding layer, and the cladding layer has a height from the substrate substantially equal to that of the insulating layer.

In one embodiment, the Mach-Zehnder waveguide modulator is disposed within a cavity of a silicon-on-insulator layer which is disposed above the substrate.

In one embodiment, the left and right optical waveguides are formed of SiGe having a first composition, and the cladding layer is formed of the SiGe having a second composition different to the first composition.

In one embodiment, the optoelectronic device is formed on a silicon-on-insulator wafer including a substrate, an insulating layer, and a silicon-on-insulator layer, the silicon-on-insulator layer including: a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator including: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide, wherein each of the left and right optical waveguides includes a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion; and a cladding layer, formed of a different material to the material of the insulating layer, on top of the substrate and beneath the junction region of the left optical waveguide or the junction region of the right optical waveguide, or both, wherein the cladding layer has a refractive index which is less than a refractive index of the respective junction region(s) such that an optical mode of the optoelectronic device is confined inside the respective junction region(s), and wherein the insulating layer does not extend below the respective junction region(s).

According to an embodiment of the present invention there is provided an optoelectronic device, including: a substrate; a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator including: a left arm including a left optical waveguide, and a right arm including a right optical waveguide, wherein each of the left and right optical waveguides includes a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion; and an epitaxial crystalline cladding layer, on top of the substrate and beneath the junction region of the left optical waveguide and/or the junction region of the right optical waveguide, wherein the epitaxial crystalline cladding layer has a refractive index which is less than a refractive index of the respective junction region(s), such that optical power is confined to the respective junction region(s).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows a plan view of an optoelectronic device;

FIG. 2A shows a cross-sectional view of the device of FIG. 1 along the line A-A′;

FIG. 2B shows a cross-sectional view of the device of FIG. 1 along the line B-B′;

FIGS. 3A-3P show various manufacturing steps;

FIG. 4 shows a variant device;

FIG. 5 shows a variant device;

FIG. 6 shows a variant device;

FIG. 7 shows a variant device; and

FIGS. 8-14 show variant structures for the optically active region and/or device.

DETAILED DESCRIPTION

FIG. 1 is a plan view of an optoelectronic device 104 as disposed on a chip 100. An input waveguide 101 is operable to guide a light signal along direction 102 and through an interface 103 into the device. The interface between the input waveguide and the device is at an angle α₁ relative to the guiding direction 102 of the light. The angle α₁ may take a value between 0° and 10°. In some embodiments α₁ is approximately 8°.

The light signal, having passed through the interface into the device 104, enters an optically active region (OAR) 105 where it may be processed or modified. For example, the optically active region may be any of: a photodiode; an electro-absorption modulator; or an avalanche photodiode. Depending on the nature of the optically active region, the light signal may then exit the OAR and device 104 via interface 108, into an output waveguide 106.

The output waveguide 106 guides light in direction 107, and the interface 108 may be at an angle α₂ relative to the guiding direction 107 of the light in the output waveguide. As with angle α₁, the angle α₂ may take a value between 0° and 10°. In some embodiments α₂ is approximately 8°, and is generally equal to α₁.

FIG. 2A is a cross-sectional view of the device 104 shown in FIG. 1, along the line A-A′. The device 104 comprises a silicon substrate 201 which is a lowermost layer of the device. Disposed on top of the substrate are two buried oxide (BOX) layers 202 a and 202 b; and, between the buried oxide layers, is a cladding layer 203 which may be Si or SiGe. On top of the cladding layer is an optically active region 105 which is connected on either side to the input waveguide 101 and output waveguide 106. The interfaces 103 and 108 between the waveguides and the OAR are shown. A capping SiO₂ layer 206 is shown in this figure. Notably, the buried oxide layers 202 a and 202 b do not extend under the optically active region 105. The buried oxide layers may extend partially under a slab of the rib waveguide, i.e. under doped regions 210 and 211.

FIG. 2B is across-sectional view of the device 104 shown in FIG. 1, along the line B-B′. This figure shows in more detail an example of the optically active region 105. Disposed on top of the cladding layer is a waveguide that comprises an intrinsic part 205, a first doped region 208 and a second doped region 209 which are formed from the same material as the intrinsic part 205. Alternatively, either doped regions 208 or 209, or both doped regions 208 and 209, may be formed from a different material from the intrinsic part 205 such as Si or SiGe. The doped regions 208 and 209 extend along an upper surface of the cladding layer 203, and up sidewalls of the intrinsic part 205. The dopants in the first doped region are of a different species to the dopants in the second doped region.

A first portion 210 of the first doped region 208 is heavily doped in comparison to the remaining first doped region. This portion 210 is connected to an electrode 232 a, which extends through the SiO₂ capping layer 206. Similarly, a second portion 211 of the second doped region 209 is heavily doped in comparison to the remaining second doped region. This portion 211 is connected to a second electrode 232 b, which extends through the capping layer 206. The OAR 105 is generally located in a cavity of a silicon layer, the cavity being partially defined by silicon sidewalls 207 a and 207 b. The intrinsic part 205 in this example is undoped, and so the OAR can be described as a p-i-n junction. As the intrinsic part 205 extends away from the cladding layer, it may be described as a proud or rib waveguide where the rib is provided by the intrinsic part 205 and a part of first 208 and second 209 doped regions which extend up the side of the intrinsic part 205 and the slab is provided by a part of the doped regions 208 and 209 which extends along the upper surface of the cladding layer 203. The rib waveguide may have a height of around 2.8 μm as measured from the upper surface of the cladding layer, and the slabs may have a height of around 200 nm. The width of the rib waveguide (i.e. the horizontal distance between the parts of the first and second doped regions which extend up the side of the intrinsic part 205) may be around 0.8 μm. The cladding layer may be approximately 400 nm thick (i.e. as measured from the uppermost surface of the silicon substrate to the uppermost surface of the cladding layer). In such examples, the coupling efficiency from the input waveguide into the waveguide 205 has been computed as approximately 99% for TE mode and 98.7% for TM mode.

The cladding layer 203 functions to confine light signals entering the OAR into the rib waveguide. It does so primarily by being formed of a material having a refractive index which is less than that of the OAR. For example, the cladding layer may be formed of a silicon layer which may be epitaxially grown or deposited using chemical vapour deposition which can have a refractive index of 3.3 to 3.8. In. In contrast, the waveguide and/or OAR may be formed primarily of silicon germanium (SiGe) which can have a refractive index of 4.0-4.7. This change in refractive index across the interface between the OAR and cladding layer may provide enough index contrast (i.e. Δn) to confine the light signals to the waveguide. It is notable that good confinement can be achieved without a buried oxide layer below the OAR, as discussed above.

FIG. 3A-3P discuss manufacturing steps to provide a device as shown in FIGS. 2 and 3, shown along the cross-section B-B′. In a first step, shown in FIG. 3A, a silicon-on-insulator wafer is provided. The wafer comprises a silicon substrate 201, a buried oxide layer 202 disposed thereon, and a silicon-on-insulator layer 207. Next, as illustrated in FIG. 3B, a first mask 212 is disposed over a region of the silicon-on-insulator layer and then the unmasked region is etched down to the buried oxide layer 202. This results in a cavity 213 in the silicon-on-insulator layer which is partially defined by sidewalls 207 a and 207 b. Next, the buried oxide layer in between the sidewalls 207 and 207 b is etched away and the first mask is removed, resulting in a structure shown in FIG. 3C. The cavity is now at least partially defined by sidewalls of the silicon-on-insulator layer 207 a and 207 b as well as sidewalls 202 a and 202 b of the remaining buried oxide layer.

Next, as illustrated in FIG. 3D, an insulating liner 215 a and 215 b may be provided along the sidewalls 207 a and 207 b of the cavity 213. Indeed, in some embodiments there is no liner provided along the sidewalls of the cavity. The liner may extend along the top of the sidewalls 207 a and 207 b as illustrated. After the liner has been provided, a cladding layer 203 is grown onto the silicon substrate 201 as shown in FIG. 3E. The cladding layer may be an epitaxially grown semiconductor (for example silicon) layer, and may be referred to as an epitaxial crystalline layer. The liner may ensure that the cladding layer grows with a generally homogenous crystal structure, as it may only grow from the silicon substrate and not from the sidewalls. As an optional extra step after regrowing the cladding layer, the insulating liner 215 a and 215 b, which may have been provided along the sidewalls 207 a and 207 b of the cavity 213, may be removed.

After the cladding layer has been provided, the optically active region 217 is grown as shown in FIG. 3F. Prior to this step, a seed layer may be grown on top of the cladding layer. This can benefit the formation of the optically active region. The optically active region may be provided by the blanket deposition of germanium into the cavity 213. After deposition, the optically active region 217 is planarized by, for example, chemical-mechanical polishing such that an uppermost surface of the OAR is level with an uppermost surface of the liner 215 a and 215 b, as shown in FIG. 3G. If there is no liner, then the uppermost surface of the OAR would be level with the uppermost surface of the sidewalls 207 a and 207 b.

Next, as shown in FIG. 3H, a second mask 218 is provided over a portion of the OAR, and the unmasked region is etched to provide slabs 220 a and 220 b of the waveguide. The unetched region provides a rib waveguide 219 as discussed above. This completes the key manufacturing steps for providing the optically active region.

As a further step, shown in FIG. 3I, a capping layer 221 is provided over the OAR. This capping layer is sufficiently thin that dopants can be implanted into regions of the OAR through the capping layer. For example, as shown in FIG. 3J, a third mask or photoresist 222 is provided a region of the device. The unmasked region is then exposed to dopants 223 of a first species, so as to dope a region 208 of the optically active region. In this example, the dopants are injected into a region of the slab 220 a which is unmasked as well as a sidewall of the rib of the waveguide. The dopants may be, for example, boron and so the region is doped with a p type species of dopant. The third mask is then removed.

Similarly, as shown in FIG. 3K, a fourth mask or photoresist 224 is provided over a region of the device. The unmasked region is then exposed to dopants 225 of a second species, so as to dope a region 209 of the optically active region. In this example, dopants are injected into a region of the slab 220 b which is unmasked as well as a sidewall of the rib waveguide. The dopants may be, for example, phosphorus and so the region is doped with an n type species of dopant. The fourth mask is then removed.

So as to decrease the electrical resistance of the first 208 and second 209 doped regions, further doping may be performed as will be discussed. In FIG. 3L, a fifth mask or photoresist 226 is disposed over a region of the device, and an unmasked region is exposed to further dopants 227 of the first species. This results in a first heavily doped region 210 within the first doped region 208. This region may be described as p++ doped relative to the p doped region 208. The fifth mask is then removed. Similarly, as shown in FIG. 3M, a sixth mask or photoresist 228 is provided over a region of the device, and an unmasked region is exposed to further dopants 229 of the second species. This results in a second heavily doped region 211 within the second doped region 209. This region may be described as n++ doped relative to the n doped region 209. The sixth mask is then removed.

As a further step shown in FIG. 30, a seventh mask 230 may be provided over a region of the device, and the unmasked regions may be etched so as to remove portions of the capping layer 221 above the first 210 and second 211 heavily doped regions. This produces vias 231 a and 231 b. The seventh mask is then removed. In a final step, shown in FIG. 3P, electrodes 232 a and 232 b are provided which respectively contact the first 210 and second 211 heavily doped regions through the vias. An electric potential can be applied via electrodes 232 a and 232 b, resulting an electric field which passes horizontally across the waveguide 219. The device may therefore utilize the Franz-Keldysh effect to modulate the amplitude of light signals passing through.

A variant device is shown in FIG. 4, where a germanium seed layer 401 is disposed between the cladding layer 416 and the silicon substrate 201. As is also shown in this figure, the seed layer 401 is positioned within a cavity of the silicon substrate 20, such that the cladding layer 416 is disposed in a similar position to previous embodiments. Like features are indicated by like numerals. As will be appreciated, the additional features shown in FIG. 2B may also be present in this device, but for the sake of clarity are not shown.

Similarly, a further variant device is shown in FIG. 5. Here, the first doped region 501 extends only part of the way up the sidewall of the waveguide 219. As will be appreciated, the additional features shown in FIG. 2B may also be present in this device, but for the sake of clarity are not shown. This device is suitable for providing a bias across the junction to enable control of the phase of light traveling through the junction region via dispersion. As used herein, a “junction region” of a waveguide is a region of the waveguide that contains a semiconductor junction. The structure of the device and its method of manufacture are similar to that disclosed in WO 2016/0139484 titled “Waveguide Modulator Structures”, the entire contents of which is incorporated herein by reference.

Another variant device is shown in FIG. 6. Here, a further silicon layer 601 is doped to provide a first doped region 602 and a second doped region 604. Alternatively, the further silicon layer 601 is not used and the first doped region 602 is made in the cladding layer 203. They respectively include a first heavily doped region 603 and a second heavily doped region 605. In contrast to the previous devices, the first doped region 602 does not extend up a sidewall of the waveguide 219, but instead extends along a lowermost surface of the waveguide 219. Further, the second doped region 604 extends along an uppermost surface of the waveguide 219. Therefore, when a voltage is applied to electrodes 232 a and 232 b, a vertical electric field can therefore be provided across the waveguide 219 in contrast to the horizontal electric field in previous examples. The structure of the device and its method of manufacture are similar to that disclosed in WO 2017/081196 A1 titled “An optoelectronic component”, the enter contents of which is incorporated herein by reference.

A further variant device is shown in FIG. 7. In this Figure, a device is shown comprising a ridge modulation region with a height h_(wg); the ridge modulation region being made up of a base 701 manufactured from a first waveguide material M₁ and a ridge 702 manufactured from a second waveguide material M₂ which is different from the first waveguide material.

The base 701 includes a first slab region extending away from a first sidewall of the waveguide ridge in a first direction, and a second slab region extending away from a second sidewall of the waveguide ridge in a second direction; the second direction being opposite the first direction.

The device includes a first doped region, the first doped region including a first doped slab region 713 a and a first doped sidewall region extending along the first sidewall of the waveguide.

As shown in the Figure, the ridge of the waveguide is formed from a lower ridge portion 712 a and an upper ridge portion 712 b. The lower ridge portion is in contact with and extends away from the base; the base and lower ridge portion both being formed from the first material M₁. The upper ridge portion is made from the second material M₂ located on top of the lower ridge portion in that it is in contact with and extends away from the lower ridge portion.

The first doped sidewall region extends along the entire sidewall of the ridge including both the lower ridge portion 712 a and the upper ridge portion 712 b. The first doped sidewall region therefore comprises a first lower sidewall portion 713 b which extends along the first sidewall at the lower ridge portion of the ridge; and a first upper sidewall portion 713 c which extends along the sidewall at the upper ridge portion of the ridge.

Similarly, at the second side of the rib waveguide, the device comprises a second doped slab region 714 a and a second doped sidewall region extending along the second sidewall of the waveguide. The second doped sidewall is made up of a second lower sidewall portion 714 b which extends along the second sidewall at the lower ridge portion of the ridge; and a second upper sidewall portion 713 c which extends along the sidewall at the upper ridge portion of the ridge.

The dopant concentration at the doped slab regions and the lower doped sidewall regions are higher than those of the upper doped sidewall regions. In the example shown, the first doped slab region and the first lower sidewall doped region are n++ doped, whilst the first upper sidewall is n doped; the n++ doped region typically contains at least one to two orders of magnitude more dopant per cm³ as compared to the n doped region. The second doped slab region and second lower sidewall doped region are p++ doped whilst the first upper sidewall is p doped.

In the example shown, the first material M₁ is formed from silicon (Si) and the second material M₂ is formed of silicon germanium (SiGe) or silicon germanium tin (SiGeSn). However, it is envisaged that the structure of this embodiment could equally be applied to other suitable optical materials. Examples of suitable dopant concentrations for an M₁/M₂ structure of Si/SiGe or Si/SiGeSn are shown in Table 1 below:

TABLE 1 Doping Doping range type [1/cm³] n 1e15-1e18 p 1e15-1e18 n++ 1e18-1e20 p++ 1e18-1e20

As can be seen in FIG. 7, the first doped slab region can be defined by a thickness d_(np1) by which it extends downwards into the slabs of the first material M₁. The first lower sidewall portion 713 b and second lower sidewall portion 714 b each extend upwardly away from the slab by a height h₃ which corresponds to the height of the lower portion of the ridge. These lower sidewall portions 713 b, 714 b extend into the ridge by respective distances d_(np2), d_(pp2), each of these respective distances being less than half the total cross-sectional width of the lower ridge portion, such that an undoped region separates the n++ region from the p++ region thereby forming a p-i-n junction.

An electrical contact (not shown) will be located at each of the slab regions in order to apply a bias across the junction which is formed by the doped regions. These electrical contacts will be located directly onto the slab (i.e. at the upper surface of the lab, on either side of the ridge). Typically the contacts may be equidistant from the respective sidewalls of the ridge.

The first and second upper sidewall portions 713 c, 714 c extend into the upper ridge portion of the ridge by a distance d_(n), d_(p) respectively, each of which is less than the respective distance d_(np2), d_(pp2), by which the lower sidewall portions 713 b, 714 b each extend into the lower portion 712 a of the rib waveguide. Examples of typical measurements are given (in nm) in Table 2:

TABLE 2 Geometry Tolerance h₁ [nm] 100-800 h₂ [nm] 100-400 h₃ [nm]  0-400 d_(np1), d_(np2) [nm]  50-300 d_(pp1), d_(pp2) [nm]  50-300 d_(p) [nm]  50-300 d_(n) [nm]  50-300

In this example, the waveguide device takes the form of a waveguide electro-absorption modulator (EAM). However, it is possible that the device could instead take the form of another optoelectronic component such as a waveguide photodiode (PD).

The structure of the device and its method of manufacture are similar to that disclosed in U.S. 62/429,701, the entire contents of which is incorporated herein by reference.

FIG. 8 shows an optically active region which is similar to that shown in FIG. 7. However, the region indicated within dotted line 801 which includes the first doped slab portion 713 a, the first lower sidewall portion 713 b, and the first upper sidewall portion 713 c is formed of crystalline or amorphous silicon. Whilst not shown, it is possible that there is a buried oxide layer beneath the base 701 but this is optional.

FIG. 9 is similar to FIG. 8, except that the region indicated within dotted line 901 now includes the second doped slab portion 714 a, the second lower sidewall portion 714 b, and the second upper sidewall portion 714 c and so these regions are also formed of crystalline or amorphous silicon. Whilst not shown, it is possible that there is a buried oxide layer beneath the base 701 but this is optional.

FIG. 10 shows a variant optically active region to those previously. Here, the entire slab, as well as a portion of the ridge 713 b 714 b, is within the region indicated within dotted line 1001. This region is formed of crystalline or amorphous silicon. This device is shown with an optionally buried oxide layer 1002 below the region 1001. This is also true of the devices shown in FIGS. 7, 8, 9, 11, and 12.

FIG. 11 shows an optically active region which is similar to that shown in FIG. 10. However here the dotted region 1101 extends up one side of the rib waveguide and so includes the first upper sidewall portion 713 c and so this is also formed of crystalline or amorphous silicon. FIG. 12 is an extension of this, where the second upper sidewall portion 714 c is also included and so this is also formed of crystalline or amorphous silicon.

FIG. 13 shows a further alternative example of the optoelectronic device. Here, as with previous embodiments, a slab 1301 and rib 1302 form a ridge waveguide disposed above a silicon substrate 201. However, in contrast to previous examples, the cladding layer 203 is not as wide as the slab 1301. Instead, first and second portions of a buried oxide layer 202 a and 202 b are disposed underneath the slab 1301.

FIG. 14 shows a further alternative example of the optoelectronic device. Generally, this example is similar to any previous example discussed (and so may have, where appropriate, any of the features disclosed with reference thereto). A difference however, shown in FIG. 14, is that this device is provided on a double silicon-on-insulator wafer. Therefore the cladding layer 1401, which confines an optical mode of the device to the optically active region 219, is above a substrate 1402 (which is generally formed of silicon). That substrate 1402 is above a buried oxide layer 1403, for example SiO₂, which is in turn above a second substrate 1404. The manufacturing steps described above apply equally here, where etching is performed at least to a buried oxide layer no longer shown in FIG. 14 but which would have been above substrate 1402.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

LIST OF FEATURES

-   100 Chip -   101 Input waveguide -   102,107 Light guiding direction -   103 Input waveguide/OAR interface -   104 Optoelectronic device -   105, 205 OAR -   106 Output waveguide -   108 OAR/Output waveguide interface -   201 Silicon substrate -   202 a, 202 b Buried oxide -   203, 416 Cladding layer -   206 Capping layer -   207 a, 207 b Silicon-on-insulator layer -   208 First doped region -   209 Second doped region -   210 First heavily doped region -   211 Second heavily doped region -   212 First mask -   213 Cavity -   214 Upper surface of substrate -   215 a, 215 b Insulating liner -   217 Grown optically active region -   218 Second mask -   219 Ridge of rib waveguide -   220 a, 220 b Slabs of rib waveguide -   221 Capping layer -   222 Third mask -   223 First dopant implantation -   224 Fourth mask -   225 Second dopant implantation -   226 Fifth mask -   227 Third dopant implantation -   228 Sixth mask -   229 Fourth dopant implantation -   230 Seventh mask -   231 a, 231 b Via opening -   232 a, 232 b Electrodes -   401 Seed layer 

What is claimed is:
 1. An optoelectronic device, comprising: a substrate; a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide, wherein each of the left and right optical waveguides comprises a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions; and crystalline cladding layer, on top of the substrate and beneath the junction region of the left optical waveguide and/or the junction region of the right optical waveguide, wherein the crystalline cladding layer has a refractive index which is less than a refractive index of the respective junction region(s), such that optical power is confined to the respective junction region(s).
 2. An optoelectronic device according to claim 1, wherein the junction region of each of the left and right optical waveguides comprises a PIN region, the junction formed from: a first semiconductor region corresponding to either a P-doped region or an N-doped region; a second semiconductor region corresponding to the other of the P-doped or N-doped region; and a central SiGe waveguide region.
 3. An optoelectronic device according to claim 2, wherein the first semiconductor region of the left optical waveguide is integral with the first semiconductor region of the right optical waveguide in a region between the left and right arms, forming a common doped region.
 4. An optoelectronic device according to claim 3, wherein the plurality of electrodes includes a common electrode located at the common doped region.
 5. An optoelectronic device according to claim 2, wherein the first semiconductor region of the left optical waveguide is electrically isolated from the first semiconductor region of the right optical waveguide by a central isolated region between the left and right arms.
 6. An optoelectronic device according to claim 2, wherein: one of the first or second semiconductor regions includes a vertical doped portion which extends along the side of the waveguide.
 7. An optoelectronic device according to claim 6, wherein the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped portion.
 8. An optoelectronic device according to claim 6, wherein the vertical doped portion extends in the vertical direction along the entire side of the waveguide.
 9. An optoelectronic device according to claim 2, comprising: an intervening lightly P-doped semiconductor region located between the P-doped semiconductor region and the central waveguide region; wherein the intervening lightly P-doped semiconductor region has a lower dopant concentration than the P-doped semiconductor region.
 10. An optoelectronic device according to claim 2, comprising: an intervening lightly N-doped semiconductor region located between the N-doped semiconductor region and the central waveguide region; wherein the intervening lightly N-doped semiconductor region has a lower dopant concentration than the N-doped semiconductor region.
 11. An optoelectronic device according to claim 2, wherein: the central waveguide region is composed of SiGe and the doped semiconductor regions are composed of Si or SiGe.
 12. An optoelectronic device according to claim 2 wherein, for each waveguide arm: the first semiconductor region includes a first lateral portion extending laterally away from the waveguide wall on a first side of the waveguide; the second semiconductor region includes a second lateral portion extending laterally away from the waveguide wall on a second side of the waveguide; and the plurality of electrodes comprises: a first electrode located directly on top of the first lateral portion; and a second electrode located directly on top of the first lateral portion.
 13. An optoelectronic device according to claim 1, wherein the junction of each of the left and right optical waveguides comprises a PN junction, the PN junction formed from: a first semiconductor region corresponding to either a P-doped region or an N-doped region; and a second semiconductor region corresponding to the other of the P-doped or N-doped region.
 14. An optoelectronic device according to claim 1, configured to operate in a push-pull mode.
 15. An optoelectronic device according to claim 1, wherein the cladding layer is formed of silicon or SiGe.
 16. An optoelectronic device according to claim 1, further comprising: an insulating layer, disposed on a first and/or second horizontal side of the cladding layer, wherein the cladding layer has a height from the substrate substantially equal to that of the insulating layer.
 17. An optoelectronic device according to claim 1, wherein the Mach-Zehnder waveguide modulator is disposed within a cavity of a silicon-on-insulator layer which is disposed above the substrate.
 18. An optoelectronic device according to claim 1, wherein the left and right optical waveguides are formed of SiGe having a first composition, and the cladding layer is formed of SiGe having a second composition different from the first composition.
 19. An optoelectronic device, formed on a silicon-on-insulator wafer comprising a substrate, an insulating layer, and a silicon-on-insulator layer, the silicon-on-insulator layer comprising: a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide, wherein each of the left and right optical waveguides comprises a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions; and a cladding layer, formed of a different material from the material of the insulating layer, on top of the substrate and beneath the junction region of the left optical waveguide or the junction region of the right optical waveguide, or both, wherein the cladding layer has a refractive index which is less than a refractive index of the respective junction region(s) such that an optical mode of the optoelectronic device is confined inside the respective junction region(s), and wherein the insulating layer does not extend below the respective junction region(s).
 20. An optoelectronic device, comprising: a substrate; a Mach-Zehnder waveguide modulator, the Mach-Zehnder waveguide modulator comprising: a left arm including a left optical waveguide, and a right arm including a right optical waveguide, wherein each of the left and right optical waveguides comprises a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions; and crystalline cladding layer, on top of the substrate and beneath the junction region of the left optical waveguide and/or the junction region of the right optical waveguide, wherein the crystalline cladding layer has a refractive index which is less than a refractive index of the respective junction region(s), such that optical power is confined to the respective junction region(s). 